The present invention relates to structural parts and to methods of forming low density core parts and, more particularly, to low density core metal parts and methods of simultaneously pressure bonding low density core metal parts and densifying the core material to provide low density core metal parts having a predetermined density and shape.
Metallic composites or sandwich structures have been used in a number of applications. These structures generally involve the bonding of two or more metal layers using various bonding techniques. These techniques involve either directly bonding the metal layers to one another using heat and sometimes pressure or involve using other materials to facilitate bonding between the metal layers.
One common method of forming metallic sandwich structures involves diffusion bonding or high-temperature solid-state welding. As described in W. Lehrheuer, High-Temperature Solid-State Welding, ASM Handbook, vol. 6 (1993), diffusion bonding is used to join layers of similar or dissimilar materials by compressing and heating the surfaces to be joined in a suitable atmosphere and maintaining the temperature and pressure until the layers are joined. Diffusion bonding results in the direct bonding of metal layers and is normally characterized by the interchange of metal molecules or diffusion of molecules at the interface of the two metal layers being bonded. Generally, diffusion bonding processes involve relatively high temperatures and low pressures which are maintained over several hours.
Diffusion bonding is also often coupled with superplastic forming in the formation of the metal structures. Superplasticity is the characteristic of certain metals to develop unusually high tensile elongations with minimum necking when deformed within a limited temperature and strain rate range. Generally, superplastic forming processes include the injection of an inert gas into the sandwich structure during its formation to facilitate superplastic forming of the structure. One example of a diffusion bonding and superplastic forming process is U.S. Pat. No. 5,024,368 to Bottomley et al. Bottomley et al. describes the diffusion bonding and superplastic forming of components made from aluminum or aluminum alloys using a temperature of about 560.degree. C. and a pressure of 1000 psi for about 3 hours. Additionally, U.S. Pat. No. 5,253,796 to Stacher et al. describes forming a titanium aluminide sandwich structure using diffusion bonding and superplastic forming at a temperature of 1800.degree. F. (980.degree. C.).
A second common method of forming composite structures is hot isostatic pressing or "hipping". In hot isostatic pressing, a generally fluid tight composite structure is compressed by applying fluid pressure to the outside of the structure. Hot isostatic pressing occurs at very high temperatures (600.degree. C. to 1000.degree. C. and higher) and pressures (15 ksi) and is normally used to densify the composite structure. For example, U.S. Pat. No. 4,907,736 to Doble describes hot isostatic pressing to form a structural composite comprising metal and filamentary layers.
A third method of forming metallic structures is pressure bonding or forge welding. Pressure bonding involves actual deformation of the composite structure using relatively low temperatures and relatively high pressures. As described in W. Lehrheuer, Forge Welding, ASM Handbook, vol. 6 (1993), pressure bonding involves applying pressure to press the layers to be bonded together, heating the interface between the layers, and increasing the pressure to upset the interface. Pressure bonding has been used in various applications to form solid metal articles including rods, bars, tubes, rails, aircraft landing gear, chains and cans.
In addition to these methods, other methods such as soldering and brazing are used to bond metal layers to form composite structures but involve the use of additional adhesive substances which are placed between the metal layers to provide indirect bonding therebetween. Soldering involves the use of a low-melting alloy (below 800.degree. F. or 427.degree. C.) which acts as an adhesive but does not form an intermetallic solution with the metals being joined. Brazing is similar to soldering but the filler alloy has a higher melting temperature (above 840.degree. F. or 450.degree. C.). In both soldering and brazing, the alloy is placed between the edges or ends of the metal layers to be bonded and heated to form a bond between the metal layers.
In various industries, low density core (LDC) metal parts, such as honeycomb, foam core, and porous structural parts, have been used to provide lightweight alternatives to solid metal structures. The structures built from these lightweight parts can, like solid metal structures, carry high structural loads, withstanding bending, buckling and compression. For example, these lightweight structures have found utility in the aerospace industry, in the ship building industry, in medical devices, and in commercial applications such as in filtration, heat exchangers, electrochemical cells, heat pipes, and pumps. LDC parts are particularly effective in aerospace applications which are buckling and crippling limited, where the LDC parts need to withstand lateral pressure without buckling or crippling, e.g., wing skins, flap skins, and bulkheads. In particular, aluminum and aluminum alloy parts have found utility for these applications.
Conventional methods of forming LDC metal parts involve brazing metal face sheets to a metal core material. Nevertheless, brazing does not allow for the adjustment of the core density, especially when metallic foams are used as the core material. Furthermore, because brazing does not form a direct bond, LDC parts which are formed by brazing are subject to delamination which may be caused by interfacial oxidation. As a result, LDC parts formed by brazing may have a low degree of structural stability. Further disadvantages are described in U.S. Pat. No. 5,174,143 to Martin, which is commonly owned by the assignee of the present application. For example, bond inconsistencies and the infeasability of forming thin section parts are also disadvantages of conventional brazing methods.
Alternative direct bonding methods such as diffusion bonding and hot isostatic pressing have proven ineffective in forming LDC metal parts, specifically, foam aluminum and aluminum alloy core parts. In particular, these methods either do not allow for effective densification of the foam core material or do not provide a method of controlling the final foam core density. Furthermore, aluminum readily oxidizes to form aluminum oxide which impairs the ability of the aluminum to diffusion bond with other metal layers. Therefore, the aluminum structures must be extensively deoxidized immediately before bonding to form effective bonds.